moved the 16x16 svg icons in the 16x16 directory
fixed a warning
removed unused functions
removed use of a deprecated function
window: put the infobar below the statusbar
sidebar: removed some extra space
do not show a low-resolution preview before loading the image

probably this gives a sense of slowness to the user
preload only 4 images to save some memory

and changed the code to allow different values for the number of
forward and backward preloaders
fixed left spacing for the filter-bar and location-bar
removed the frame from the thumbnail list

it is not needed with the dark theme
thumbnail list: make the selected images more highlighed
always use CAIRO_FILTER_FAST for painting the surface

the high quality scaling is not perfomed by cairo
added code to optionally force the original image loading

for debugging purpose
image viewer: image quality not updated sometime
do not update the image if the current quality is better
use a shorter timeout to update the image quality
removed frame border from the empty thumbnail list
toolbox: removed frame border, added header separator
fixed black sidebar for the sharpen image tool
make code more robust